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Some Addition to Neutron Porosity Logging edited by P. Vass
Some Addition to Neutron
Porosity Logging
edited by P. Vass
Neutron Porosity Logging
The measurement of porosity is based on the high slowing-down
power of hydrogen.
If a hydrogen rich fluid (water, oil) fills the pore space, the energy
degradation of neutrons principally depends on the porosity.
But the hydrogen nuclei play an important role not only in the
slowing-down of neutrons but also in the thermal neutron capture
at lower energies (in the absorption phase).
If the concentration of elements having high value of microscopic
thermal capture cross section (chlorine, gadolinium, boron, lithium)
can be neglected in the formation, the rate of thermal neutron
capture mainly depends on the liquid filled porosity.
The probability of thermal neutron capture to occur increases with
the thermal neutron density (number of thermal neutrons per unit
volume, [neutrons/cm3]) of the investigated volume.
The flux of gamma ray coming from thermal neutron capture also
increases with the thermal neutron density.
Neutron Porosity Logging
The relationship between thermal neutron density and liquid filled
porosity (that is the hydrogen concentration) depends on the
source-detector spacing.
For short distances the thermal neutron density in the medium
increases with the porosity.
Beyond a certain distance the relationship becomes inverse.
The detectors applied in the logging tools are placed in the far zone
where the inverse relationship is valid.
Accordingly, the increase of liquid-filled porosity decreases both the
flux of thermal neutrons and the flux of capture gamma rays near
the detector. Thus, the detector count rate will be less.
The increase of shale or clay content also reduces the count rate in
the far zone because of the high bound water saturation.
Neutron Porosity Logging
The figure shows the distribution of
thermal neutron density as a function
of distance from the source in a
homogeneous medium.
The different curves of the graph are
pertinent to rocks with different
porosities in the range of 10 to 40%.
The thermal neutron density quickly
decreases with the distance in all
cases.
The curves intersect each others at
about 20-25 cm from the source. Here,
the thermal neutron density is just
slightly dependent on the porosity.
In the near zone, the thermal neutron
density increases with the porosity.
In the far zone, the thermal neutron
density decreases with the porosity.
Neutron Porosity Logging
There are three different ways of obtaining porosity information
depending on the type of detected particles or ray.
The detection of following particles and ray is applicable:
• epithermal neutrons,
• thermal neutrons,
• and prompt gamma rays coming from thermal neutron capture.
Accordingly, three versions of neutron porosity logging were
developed:
• neutron-epithermal neutron logging,
• neutron-thermal neutron logging,
• and neutron-gamma (ray) logging.
Neutron Porosity Logging
The temporal separation of different phases (slowing-down, diffusion
and absorption) also appears as the spatial separation of neutron
populations having different energy intervals within the formation. The
order of zones according to increasing distance from the source:
epithermal neutrons thermal neutrons gamma ray from thermal
neutron capture. Thus, an optimal source-detector spacing must be
selected for the type of particle or ray to be detected.
The difference in source-detector spacing for the three neutron
porosity methods (left: neutron-epithermal neutron, middle: neutron-
thermal neutron, right: neutron-gamma).
O. Serra, L. Serra: Well Logging, Data Acquisition and Applications
Neutron Porosity Logging
Qualitative comparison of some properties of the three neutron
porosity methods:
neutron-
epithermal
neutron
neutron-thermal
neutron
neutron-gamma
source-detector
spacing
short medium long
minimum bed
resolution
good medium bad
depth of
investigation
shallow medium deeper
sensitivity to
thermal neutron
absorbers
not sensitive sensitive very sensitive
Neutron Porosity Logging
The neutron-gamma logging method was developed at first, but it is
not used any longer.
Its high sensitivity to thermal neutron absorbers means that the
detected count rate of gamma ray depends on not only the
hydrogen concentration but also the concentration of additional
elements (e.g. chlorine). Since this additional effect does not carry
porosity infromation, the porosity estimation becomes less reliable.
In addition, the background gamma radiation of rocks also
influences the detected gamma count rate.
Neutron-epithermal neutron tools (also called side-wall neutron
tools) are the least sensitive to thermal neutron absorbers, because
the detected neutrons have too high energies to be captured by
thermal neutron absorbers.
However, their radial depth of investigation is rather shallow due to
the short source-detector spacing.
Neutron Porosity Logging
Another disadvantage is the worse efficiency of epithermal neutron
detectors (the uncertainty of measured count rate is higher).
Neutron-thermal neutron tools provide the best compromise
between the above-mentioned advantageous and disadvantageous
properties.
This is the reason why their usage is common in well logging.
Neutron Porosity Logging
The dual-detector system of neutron
– thermal neutron logging tools is
called compensated neutron logging
tool (CNL).
Due to the suitable selection of
detector positions the ratio of near
to far detector count rates primarily
depends on the slowing-down
length of the formation (which is
explicitly affected by the hydrogen
concentration).
Thus, the effects of thermal neutron
absorbers, mud cake and borehole
irregularities are significantly
reduced.
Darwin V. Ellis, Julian M. Singer:
Well Logging for Earth Sciences
Neutron Porosity Logging
The typical vertical resolution of a compensated neutron logging
tool is about 2 ft (~61 cm). Some tools can provide a vertical
resolution of 1 ft. (The vertical resolution is closely related to the
minimum bed resolution)
The radial depth of investigation decreases with the porosity.
Under average conditions the depth of investigation is about 10 in
(25.4 cm).
The sensitivity of neutron logging to the formation porosity
decreases with the increase of porosity in the range of 2 to 40 %.
It also decreases below 2% since the effect of rock matrix will
predominate over the effect of hydrogen with respect to the
slowing down of neutrons.
The porosity range in which the neutron-thermal neutron logging
is able to provide reliable measurement spans the interval of 2%
to 35%.
Neutron Porosity Logging
Standard or traditional presentation of neutron porosity log
together with the density log
The two log curves (or traces) are presented in the same log track
with linear scales. Typically, the third track (from the left side) is
reserved for them, but they can also occupy two tracks (track 2
and 3) when resistivity curves are not displayed in track 2.
The scale of density log curve shows from left to right, and the
scale of neutron porosity curve is reversely directed.
The dynamic range of density log scale is 1 g/cm3. This change
in bulk density corresponds to a porosity change of 60 p.u.
(porosity unit: porosity represented as decimal fraction), so the
dynamic range of neutron porosity curve is 60 p.u.
When sandstone is assumed to be the rock matrix the scales are
usually the following:
bulk density 1.9 – 2.9 g/cm3
neutron porosity 0.45 – - 0.15 p.u. (or 45 – -15 %)
Neutron Porosity Logging
Standard or traditional presentation of neutron porosity log
together with the density log
Thus, the division of 0 p.u. corresponds to the division of 2.65
g/cm3 (the density of quartz grains) on the scales.
When limestone is assumed to be the rock matrix the scales are
usually the following:
bulk density 1.95 – 2.95 g/cm3
neutron porosity 0.45 – - 0.15 p.u. (or 45 – -15 %)
Here, the division of 0 p.u. does not exactly fit to the density of
limestone matrix (2.71 g/cm3) on the scales.
For high apparent neutron porosity values (typical in shaly
sandstone formations) the following scales are also used:
bulk density 1.65 – 2.65 g/cm3
neutron porosity 0.6 – 0 p.u. (or 60 – 0 %)
Neutron Porosity Logging
Of course, the application of these scales are not compulsory.
The display scales can be easily modified with petrophysical
software products.
What we should care for, is to adjust the scales of the two log
curves so that they agree, or overlay, when the permeable
formation is clean (non-shaly) and water-filled.
Darwin V. Ellis, Julian M. Singer: Well Logging for Earth Sciences
